Anisotropic proton hyperfine interaction in the electron spin resonance spectra of the n-propyl and n-butyl radicals in an argon matrix at 4.2 K

The electron spin resonance spectra of the n‐propyl and n‐butyl radicals have been measured in an argon matrix at 4.2 K. The radicals were produced in situ by photolysis of the corresponding iodides. The spectra have been analyzed fully and the analysis confirmed by comparison with simulated spectra obtained by a comprehensive computer program. From comparison of the simulated spectra and the experimental ones the following hyperfine interactions and g tensors have been obtained: $A_1^{\alpha }\text{=29.2± 0.05\hspace{0.17em}}\mathrm{G}$ ; $A_2^{\alpha }\text{=17.0± 0.05\hspace{0.17em}}\mathrm{G}$ ; $A_3^{\alpha }\text{=20.1± 0.05\hspace{0.17em}}\mathrm{G}$ ; $A_{\perp }^{\beta }\text{=37.3± 0.05\hspace{0.17em}}\mathrm{G}$ ; $A_{\parallel }^{\beta }\text{=34.0± 0.05\hspace{0.17em}}\mathrm{G}$ ; $A_{\mathrm{iso}}^{\alpha }\text{=22.1± 0.05\hspace{0.17em}}\mathrm{G}$ ; $A_{\mathrm{iso}}^{\beta }\text{=35.10± 0.05\hspace{0.17em}}\mathrm{G}$ ; g₁ = 2.0030, g₂ = 2.0033, and g₃ = 2.0025. The parameters for both radicals were the same within our experimental errors. By analysis of the linewidths observed for the β‐proton couplings we show that at 4.2 K the methylene group in these radicals is undergoing restricted reorientation or tunneling rather than classical free rotation. Modified Bloch equations for the two site case were used to estimate the correlation time for the reorientation or tunneling. We were also able to determine the equilibrium configuration of the radicals. The correlation times for reorientation of the β protons of both radicals were found to be the same within experimental error and to be equal to τ = 6 × 10⁻¹⁰ sec at 4.2 K. This fast correlation time is attributed to extensive quantum mechanical tunneling occurring at 4.2 K.